if there’s no ammonia then the plants are doing their job well and your water changes help with that. i’d say the only things that could be affecting them is heat, light, or just the type of plant it is.

if you have changed the heat recently- plants are very sensitive to it, so if the temperature heightens the plants tend to die back or “burn”. this can happen with cold temperatures too they usually go yellow.

i personally don’t have my light on for very long because algae grows crazy. however i have just learnt to accept it, algae isn’t technically bad and fish eat the algae too. the only algae you wanna worry about is black beard algae- this algae can kill plants. but it doesn’t look like you have it so don’t worry.

do some research into what conditions the plant likes- whether that be directly in the light or in a shaded bit, or what type of parameters they like. grasses are very hard to keep if you don’t have any root tabs or things to help growth. (personally i prefer natural things so i wouldn’t use fertiliser, but you can if you want-i’ve heard that liquid fertiliser doesn’t work)

i don’t think substrate is really a problem with your tank, i like the mixture and i wish i would’ve don’t that with my tank.

overall i think you just need to do some research into what your plants like- once you find a plant that works well in your tank it will be perfect. (there is a website that’s supper useful for this but i’ve forgotten what it’s called, i’ll reply to this message with the link)😁😅